They will die back at the end of summer., depending on how soon the cold arrives. Yours might be getting ready to turn in for the winter. When they do go dormant it's important to keep them pretty dry.

Do you have them sitting in water? That wouldn't be a good idea. They are more drought tolerant than they are flood tolerant. Keep the potting mix moist but not wet otherwise you'll end up rotting the tuber. If your season is still warm enough for growing then that yellow/brown could be from too much water. When the weather does get colder then yes, you can expect the foliage to dry off like that. Once the plant does die off keep the tuber dry until it starts to shoot next season. People usually take the tubers out of the soil and put them in a cool (not cold) dry cupboard or on a shelf.